1. Field of the Invention
This invention relates to an inexpensive detector of electrical arcs on power lines, for providing advance warning of potentially dangerous conditions.
2. Discussion of the Prior Art
Electrical arcs can develop temperatures well above the ignition level of most common flammable materials, and therefore pose a significant fire hazard. For example, worn power cords in the home may arc sufficiently to start a fire. Fortunately, arcing is an inherently unstable phenomena and does not usually persist long enough to start a fire. Under certain conditions, reflected in particular characteristics of the electrical disturbance produced, the likelihood of the arc persisting and starting a fire is much higher. It is one object of the present invention to provide a method and apparatus to detect such potentially dangerous arcs by monitoring voltage disturbances on the power lines.
Two basic types of arcing are likely to occur in the home: line fault arcing and contact arcing. An arc due to a line fault results from either a line-to-line short or a line-to-ground short. When a fault of this type occurs several things are likely to happen: 1) the fault will draw current up to or beyond the capacity of the circuit; 2) lights will dim indicating an excessive load is being drawn; and 3) if the circuit is properly protected by a circuit breaker, the breaker will trip, interrupting supply of power to the arc. All of this will likely occur and be completed in less than a second. The resulting arcing will be explosive but short-lived, reducing the total heat to be dissipated by surrounding materials and thus reducing the likelihood of a fire.
Contact arcing results from a high resistance connection in series with a load. This may occur due to loose connections, oxidized contacts, or foreign non-conducting material interfering with the conduction path.
One example of a condition that may cause contact arcing is a well-used wall outlet wherein the spring pressure provided by the contacts has been reduced through age and use, so that insufficient pressure is applied to the inserted plug contacts to ensure low-resistance connection.
Contact arcing is also commonly caused by use of extension cords of insufficient current-carrying capacity. For example, the plug may be heated by resistance heating, gradually decomposing elastomeric insulating material around the contacts until the material partially flows into the contact area, preventing proper contact from being made. This process may become regenerative as the initial arcing produces more heat, carbonizing the insulation, producing a hard insulative layer on the contact surface.
A third cause of contact arcing often observed in aluminum wiring involves the oxidation of contacts. In this case a chemical process, principally oxidation, builds up a semi-conductive or non-conductive layer on the surface of the contacts. Preferably, when the material of the conductors is susceptible to oxidation, the contacts are gas-tight, preventing oxygen from entering and promoting oxidation. However, if the connections become loose over time, oxidation begins and arcing can result.
Many instances of contact arcing result from the gradual degeneration of the current-carrying contacts. Dangerous arcs may begin as small occasional arcing, gradually building up over time until the arcing become persistent enough to start a fire. For this reason, it would be highly advantageous if contact arcing conditions could be detected early, and a warning provided before the fault reaches a dangerous level.
It will thus be appreciated that there are fundamental differences between line fault arcing and contact arcing. The former will generally involve high currents (&gt;20A), be somewhat explosive and either burn itself out or trip a circuit breaker. Conventional circuit protection devices are normally adequate to guard against line fault arcing. By comparison, the average current drawn in contact arcing is no more than the current drawn by the load itself. Nevertheless, even low-current contact arcing, for example, a 60 watt light bulb on the end of a faulty extension cord, or a set of Christmas tree lights with faulty contacts may release sufficient heat to cause a fire. Accordingly, conventional circuit breakers are inadequate to prevent dangerous conditions due to contact arcing.
A need therefore exists for an inexpensive plug-in monitoring device capable of detecting arcing that may result in a fire. The most convenient device would be a small `night-light` style plug-in module capable of monitoring an entire house and providing a warning if potentially dangerous arcing occurs. The detection, in this case, must be made by monitoring voltage alone; to monitor current would require a current sensor to be placed around a conductor, thereby requiring the device to be wired in place. Ideally, such an arc detector would be the electrical arc equivalent to the smoke detectors widely in use today, with the further advantage of warning of a potential fire days, weeks or even months in advance of its occurrence.
A need also exists for a circuit breaker that in addition to detecting arcing that may result in a fire removes power from the load in the event that it detects sufficient arcing to present a hazard. Such a device could be conveniently packaged in much the same style as a conventional circuit breaker or it could be installed in an outlet similar to the currently available Ground Fault Interrupters. Because the load current flows through the circuit breaker, it is convenient in this application to monitor load current.
Both arc detectors must be immune to common noise present on household power lines, e.g., due to lamp dimmers, motors, carrier-current communications systems, switches, and the like.
While there have been a number of devices proposed to detect arcing, most address arcs caused by line faults. U.S. Pat. No. 5,121,282 issued to White, for example, describes a system that monitors both line voltage and current for characteristics particular to arcing and trips a circuit breaker if enough of these characteristics are present. The White device assumes, however, that the arc is the result of a line fault. One characteristic of a line fault is that the fault current will lag the voltage by 70.degree.-90.degree. . This is because under line fault conditions, the current flow will depend almost entirely on the power distribution wiring which is generally highly inductive. A plug feeding a heater that is arcing in the socket--that is, exhibiting a contact fault--will not exhibit this characteristic and thus the fault will not be detected by the White device.
U.S. Pat. No. 4,639,817 to Cooper et al shows an arc detector for "grid" or "spot" type power networks as used in large commercial or industrial installations. The Cooper circuit interrupts the power if high-frequency (10 KHz-100 KHz) noise of 14 more than a threshold amplitude is detected for more than 0.7 seconds. If adapted to home use, this detector would be tripped by continuous high-frequency noise, such as from electric drills and the like.
U.S. Pat. No. 4,858,054 to Franklin recognizes that arc short circuits differ from dead short circuits, as described above, and indicates that different detection techniques should be employed. However, Franklin's device still monitors the current and trips only when current in excess of a predetermined level is detected. This level of current must be much higher than the circuit's rated current, to avoid tripping on motor start up currents and the like. Accordingly, Franklin's device can only detect arcs in short circuits, and cannot detect a contact arc in series with a current-limiting load.
Also of general interest are U.S. Pat. Nos. 5,038,246 to Durivage, 4,951,170 to Fromm, and 4,402,030 to Moser et al.
One device currently available is the Ground Fault Interrupter or GFI. Typical GFI devices are capable of detecting leakage currents to ground as low as several milliamps, and trip an associated circuit breaker in response. A GFI very effectively reduces not only the danger of fire due to shorts to ground but also protects humans that may be in the electrical path. A GFI device is not however capable of monitoring contact arcing as discussed herein.
It should be understood that while a distinction is made herein between line fault arcing and contact arcing for purposes of clarity, the arc detector of the present invention may be applicable to detection of arcs due to both conditions. For example, when a relatively high-resistance line fault occurs, it may exhibit substantially the same characteristics as described for contact faults.